For valves in critical service applications it is usually very important that the valve does not leak to the environment, particularly where toxic gases or other fluids which may be especially harmful are involved. Such leakage frequently occurs between the valve stem and valve bonnet and may be the result of destruction of the valve stem seal from such causes as unusually high pressures or temperatures as are associated with a fire, corrosive attack on the seal from the effluent flowing through the valve, or the gradual degradation of the seal from repeated valve cycling as might be due to galling of the valve stem or the valve stem bore by interaction with metallic packing rings or the erosion of the sealing surfaces of the valve stem packing from repeated use which is a common failure associated with rotary motion valve stems.
The various stem packing materials and packing assemblies which have been conventionally used with valves for critical service applications have not proven to be entirely satisfactory in all respects. Graphite and asbestos, which have been extensively used with "fire-safe" valves, do not generally provide for a long cycle life and asbestos itself may also be a hazard to the environment. Polytetrofluoroethylene and other plastic materials are widely used as packing materials but the application of high pressure to plastic packings tends to severely extrude the packing so that it is either dissipated or functionally invalidated. The sealing capabilities of these materials can also be severely degraded by erosion of their sealing surfaces by repeated cycling of the valve stem or by structural degradation of the packing by exposure to high temperature fluids or the elevated temperatures associated with a fire. Many packing arrangements include metallic rings, but these frequently lead to galling of the sealing surfaces and a loss of sealing capability.